The present application relates to load-current-controlling circuitry and, more particularly, to a novel phase control circuit for operating a lower voltage resistive load from a higher voltage A.C. line.
It is often desirable to operate a lower-voltage load from a higher-voltage source periodic waveform. Typically, such loads are resistive and have a significant resistive temperature coefficient, whereby use of phase control circuitry for controlling the magnitude of load current will place relatively high stress on switching devices in series with the load. Thus, when a load, such as a lamp, resistance heater element and the like, is to be energized from A.C. mains, but requires less than the full mains voltage thereacross for proper operation, a power switching device in series with the load, across the lines, will be frequently subjected to inordinately high stress and may be damaged. Similarly, if the power device is to be rendered conductive only for a portion of the source waveform cycle, the switching device should be turned on at a proper point in the cycle and remain on for a consistent portion of the cycle. Therefore, loss of synchronization with the source waveform may cause the switching device to either turn on at the wrong time or to turn on for an excessively long time. In either case, load resistance, and therefore load power, is not controlled and the load and/or switching device may be damaged. If the required voltage down-conversion is large, on the order of 4:1 or 5:1, the load may not survive a full source waveform half-cycle of conduction, and it is desirable to absolutely preclude such conduction under improper conditions. It is therefore highly desirable to provide a phase control circuit capable of energizing a low voltage load directly from a higher-voltage (A.C. line) source waveform with controllable resistance and with phase control and switching performed in absolute synchronization with the line waveform. It is also desirable to provide instantaneous shut-down of the series current-conductive device if synchronization with the line waveform is lost. It is similarly desirable to provide for a gradual increase in load current at load start-up, to limit inrush current and prevent load damage.